This invention relates to axial flow fans, for example, fans designed for operation in the flow exiting a heat exchanger or in free air (e.g., a room fan).
Axial flow fans generally have a set of blades, each of which is secured at a root end to a hub that is driven by a rotating shaft and from which the blade extends radially outwardly. The blades are generally "pitched" at an angle to the plane of fan rotation to generate an axial air flow as the blades rotate.
Fan designs must accommodate a number of diverse considerations. For example, automotive fans situated behind a radiator should be compact (due to space limitations in the engine compartment), efficient (avoiding wasted energy which directs air in turbulent flow patterns away from the desired axial flow), relatively quiet, and strong (to withstand the considerable forces generated by air flows and centrifugal forces).
Generally fan blades are "unskewed"; that is, a radial center line of the blade is straight, and the blade chords perpendicular to that line are uniformly distributed about the line.
In one fan described in my U.S. Pat. No. 4,358,245, which is hereby incorporated by reference, the fan blades are highly forwardly skewed, i.e., the blade center line curves in the direction of fan rotation as it extends from the root to the tip of the blade. The blade angle in the fan disclosed in U.S. Pat. No. 4,358,245 increases over the outer 30% of the blade.
Applicant is aware of some fans which use backwardly-skewed blades. For example, General Motors Corporation has used a fan with a modest backward skew on its "X-Car". The blade angle of that fan increases with increasing diameter along the outer portion of the blades, and the skew angle at the blade tip is approximately 40 degrees. Other fans with back-skewed blades include room fans or transformer cooling fans, which also have an increasing blade angle with increasing diameter, and which generally do not have an outer ring or band connecting the blade tips.
Various fans employ an outer ring or band which is coaxial with the hub and is secured to the outer ends of the blades. For example, my above-mentioned U.S. Pat. No. 4,358,245 shows such a ring, as do other patents described therein.